The invention is directed to a programmable disc drive actuator controller and method for correcting a first transducer or head that is not reading correctly by applying a write current thereto, and, more particularly, to a programmable controller and method that uses a different transducer to determine the position of the incorrectly reading transducer before a write current is applied to the first transducer so that useful data is not overwritten by the first transducer.
Modern hard disc drives include one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks by an array of transducers or heads mounted for movement to an electronically controlled rotary actuator assembly.
The heads are mounted via flexures at the ends of a plurality of arms which project radially outward from a substantially cylindrical actuator body. The actuator body is journaled by ball bearing assemblies to rotate about a pivot shaft which is mounted to the disc drive housing at a position closely adjacent to the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs and the heads move in a plane parallel with the surfaces of the discs.
The actuator voice coil motor includes a coil mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of an array of permanent magnets. When controlled DC current is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnets and causes the coil to move in accordance with the well-known Lorentz relationship. As the coil moves relative to the magnets, the actuator body pivots about the pivot shaft and the heads are moved across the disc surfaces.
Typically, the heads are supported over the discs by slider assemblies which include air-bearing surfaces designed to interact with a thin layer of moving air generated by the rotation of the discs, so that the heads are said to xe2x80x9cflyxe2x80x9d over the disc surfaces. Generally, the heads write data to a selected data track on the disc surface by selectively magnetizing portions of the data track through the application of a time-varying write current to the head. In order to subsequently read back the data stored on the data track, the head detects flux transitions in the magnetic fields of the data track and converts these to a signal which is decoded by a read channel circuit of the disc drive. For a detailed discussion of disc drive reading and writing techniques, see U.S. Pat. No. 5,276,662 entitled DISC DRIVE WITH IMPROVED DATA TRANSFER APPARATUS, issued Jan. 4, 1994 to Shaver et al., assigned to the assignee of the present invention.
When uncorrectable read errors are detected, the disc drive may reread the data and, if the read error persists, the disc drive may attempt other corrective operations, such as seeking away, which involves moving the head away from, and then back to, the selected track and rereading the data, or modifying the channel parameters used by the read channel and rereading the data. Disc drives are commonly provided with a defined error recovery procedure in the event of read errors so that a predefined sequence of corrective operations is employed upon occurrence of such errors.
Magnetoresistive heads display certain magnetic domain instabilities that render the head unable to adequately read. Not only does this cause data read type errors but the head is also unable to recover servo information under this condition. The inability to recover servo information prevents the servo system from staying on track when the failing head is selected.
A write operation is often useful in bringing a head out of this condition. Passing a write current through the head tends to re-randomize the magnetic domains and allow the head to read. However, for embedded servo type drives, the failing head cannot be used to track follow. A write current or xe2x80x9cdummy write,xe2x80x9d whose only purpose is to pass write currents through the head to stabilize the head must be done in an area where no useful data is stored.
In older, dedicated servo products, such an operation was trivial. In an embedded servo drive, however, the inability to read causes an inability to servo so the drive cannot track follow on the failing head in order to complete the dummy write. Thus, it is UNSAFE to apply write current to the failed head since its location is not known and therefore useful data may be overwritten. Thus, it is desirable to provide a method of correcting a head that is not reading adequately while preventing useful data from being overwritten.
Against this backdrop the present invention has been developed. A first aspect of the invention is a disc drive that includes a wiggle retry program for correcting a head in a disc drive system that is unable to read correctly. The head is corrected by applying a write current to the head. The drive includes a first head for reading data stored on a disc and means coupled to the head for determining the position of the first head when it fails to read correctly and applying the write current to the first head after the first head""s position is determined.
According to a second aspect of the invention, there is provided a method of correcting a head in a disc drive that is unable to read correctly. The method includes the steps of (a) switching to a second head when a first head fails to read, (b) receiving servo information from the second head, and (c) applying a write current to the first head when the servo information received from the second head indicates that the first head is in a safe area on a disc.
According to a third aspect of the invention, there is provided a computer-readable medium having computer executable instructions for performing the steps of the method according to the second aspect of the invention.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.